In a liquid crystal display (LCD), the performance of image display devices using an optical film is greatly affected by the performance of a backlight unit. This is because the devices are based on control of amount of light by reflecting or transmitting the light through the optical film.
Various optical films having excellent optical performance have been suggested for effective application to image display devices. Among those optical films, a prism sheet is used to improve brightness of a liquid crystal display (LCD). Since an LCD cannot emit light by itself, it obtains light from a light source (CCFL or LED) and distributes the light to an overall area through a light guide plate. Then, the light is transformed to a surface light source having more uniform brightness through a diffusive sheet. In the course of this procedure, efficiency of light emitted from an initial light source is steadily reduced. If a prism sheet is used, brightness can be increased by changing side light to front light and collecting the reflected light.
The prism sheet used as a light collecting sheet is an optical film having thin-film flexibility, which forms an optical pattern structured such that a prism pattern is arranged in a linear array at one side, thereby improving brightness.
As the index of refraction increases, the properties of the prism film are improved, thereby realizing high brightness. As representative examples of conventional resins having high index of refraction, bromine-substituted epoxy resins are often used. For example, an epoxy resin produced by adding acrylic acid to a tetrabromobisphenol A type epoxy resin and a bisphenol A type epoxy resin and then mixing styrene, divinylbenzene, benzyl methacrylate, and the like to the resulting mixture may be used.
However, the epoxy resin has an index of refraction of 1.590, which is still low. Further, the epoxy resin also exhibits a low Abbe's number of 32, which needs further improvement in order to be used for optics. When halogen-based resins (such as, bromine-substituted epoxy resins) are combusted, they can generate carcinogens, such as polyhalogenated aromatic dioxin or polyhalogenated dibenzofuran. In addition, there have been problems that gas such as hydrogen bromide, hydrogen chloride and the like generated upon combustion can be harmful for human body and environment.
In addition, there have been developed optical materials having a urethane bond or thiocarbamoyl phosphate S-alkylester bond, which are formed by adding an internal release agent to a mixture of an aromatic polyisocyanate containing a sulfur atom and active hydrogen compounds, such as a polyol and a polythiol, and subjected to injection molding. However, these optical materials have low heat stability and thus have disadvantages in that the optical products are partially deformed upon hard coating.
Further, most resins having high index of refraction as listed above are not suited to mass production and production applications due to high viscosity, high cost, purity problems, and the like.
US Patent Publication No. 2009-0220742 discloses a reflective film having improved brightness, which may include a polyacrylate resin and contains a UV curable resin comprising at least one acrylic monomer.